This invention relates to the preparation of rigid polyurethane foam and more particularly to the preparation of rigid polyurethane foam in the presence of aminofunctional polysiloxane polyoxyalkylene block copolymers to obtain rigid foam with at least partially open cells.
According to the state of the art, rigid polyurethane foams are prepared from polyols with at least three hydroxyl groups per molecule, at least difunctional polyisocyanates, catalysts, blowing agents and polysiloxane polyoxyalkylene block copolymers as foam stabilizers, as well as, if necessary, conventional additives.
A comprehensive account of the raw materials and the methods, which can be used, is to be found in Ullmann "Enzyklopaedie der technischen Chemie" (Encyclopedia of Industrial Chemistry), 1980, volume 19, pages 301 to 341 and in the Kunststoff-Handbuch (Plastics Handbook), volume VII, Polyurethanes, by R. Vieweg and A. Hoechtlen, published by Carl Hanser, Munich, 1966, pages 504 to 544.
In general, polyetherols or polyesterols with at least three hydroxyl groups per molecule are used, the OH number of the polyols generally lying between 300 and 800.
As blowing agent, preferably trichlorofluoromethane or a mixture of trichlorofluoromethane and difluorodichloromethane is used, in general in combination with water. The water reacts with the isocyanate, carbon dioxide and polyurea being split off. The resulting molded articles are almost always closed cell. The density of the rigid foam is between about 23 and 1,000 (g/L or kg/m.sup.3).
If a rigid foam is prepared with a density of less than 23 (g/L), there is generally shrinkage, that is, the framework of the closed cells can no longer withstand the external air pressure. This shrinkage is supported by the diffusion of the carbon dioxide through the intact cell membranes in the direction of the surface of the foamed object.
If dimensionally stable, very light, rigid polyurethane foams are to be prepared, excessive amounts of water, for which no corresponding amount of isocyanate is available, are used pursuant to the state of the art. On reaching the boiling range, the excess water evaporates and tears the cell membranes, unless the latter are excessively stabilized.
The so-called I-K foams are a different type of rigid polyurethane foams. The necessary polyol with the catalysts, the stabilizers and the physical blowing agent and an appreciable excess of isocyanate is added here to a spray can and the mixture is allowed to react in the spray can. A prepolymer containing isocyanate groups is formed. If the spray valve is activated, the blowing agent forces the prepolymer out of the can and there is spontaneous foaming. A very soft foam is formed at first. The isocyanate groups still present react with the moisture of the air, forming a largely closed cell, rigid foam. These types of foam are used, for example, for fastening door frames and window frames. The cavity between the wall and the object that is to be built in is thus formed. Because of fluctuations in air pressure, the closed cell foam expands or contracts; this has a disadvantageous effect on door and window frames, that is, the accuracy of fit of the doors and windows varies.
It has been ascertained that the quality and properties of the rigid foams formed depend in a large measure on the structure and chemical composition of the foam stabilizers used. For this reason, polysiloxane polyoxyalkylene block copolymers of different structure and different composition have already been described as foam stabilizers.
As polysiloxane polyoxyalkylene block copolymers for the aforementioned purpose, the German Auslegeschrift 17 19 238, for example, discloses siloxane oxyalkylene copolymers of the general formula ##STR2## (1=3 to 25; x=1 to 25; y=0 to 15; z=2 or 3; p=1 to 10; R=hydrogen or methyl), which are hydrolytically stable, with the proviso that at least 25% by weight of the oxyalkylene groups are oxyethylene groups and, in the event that R=H, the hydroxyl groups constitute at least 1.5% by weight of the copolymer.
For the preparation of polyurethane foams, the German Patent 20 29 293 discloses the use of siloxane-modified carbamic acid derivatives as foam stabilizers, which consist of at least one structural unit of the general formula ##STR3## and further structural units of the general formula EQU R.sub.2 'SiO.sub.2/2
which are linked through Si--O--Si bonds with the first-mentioned structural unit, at least one structural unit per thousand corresponding to the first-mentioned formula, wherein
R' represents an optionally halogenated or cyano-substituted C.sub.1 -C.sub.10 alkyl, C.sub.4 -C.sub.10 cycloalkyl or C.sub.6 -C.sub.10 aryl group, PA1 R" represents a hydrogen atom or a methyl or phenyl group, PA1 R'" represents a C.sub.1 -C.sub.10 alkyl, C.sub.4 -C.sub.10 cycloalkyl, C.sub.2 -C.sub.10 alkenyl, C.sub.7 -C.sub.10 aralkyl, di(C-C.sub.10 alkyl)amino-C.sub.1 -C.sub.10 -alkyl, C.sub.6 -C.sub.10 aryl, or C.sub.7 -C.sub.10 alkaryl group, PA1 Q represents a saturated C.sub.1 -C.sub.6 alkyl group or a C.sub.6 aryl group, PA1 m is 1 or 2 PA1 n is 2, 3 or 4 and PA1 b is a whole number from 1 to 200 PA1 R.sup.5 is a hydrogen group or an alkyl group with 1 to 4 carbon atoms, a polyether group of the formula --(C.sub.q H.sub.2q O).sub.r R.sup.9 (q=2, 3 or 4, r=1 to 100, R.sup.9 =a hydrogen group or an alkyl group with 1 to 4 carbon atoms) or the group ##STR5## wherein R.sup.6 is a divalent aliphatic hydrocarbon group with 2 to 6 carbon atoms or a divalent aromatic hydrocarbon group, PA1 R.sup.7 and R.sup.8 are each an alkyl group with 1 to 4 carbon atoms, which may have an OH group or be a common constituent of a 5- or 6-membered ring, which may contain an oxygen or nitrogen atom, PA1 z=0 or 1. PA1 X is a polyoxyalkylene block of the general formula EQU --R.sub.p.sup.2 --O--(C.sub.m H.sub.2m O--).sub.m R.sup.3, PA1 R.sup.3 is a hydrogen group or an alkyl group with 1 to 4 carbon atoms, PA1 p has a value of 0 or 1, PA1 m represents the number 2, 3 or 4, the average value of which is 2.0 to 2.7, PA1 n has a value of 1 to 100, PA1 Z represents X, Y or R.sup.1, PA1 a has a value of 10 to 150, PA1 b has a value of 1 to 20, PA1 c has a value of 1 to 20.
the groups and numerical values at each site of a molecule being independent of one another.
Further, particularly suitable polysiloxane polyoxyalkylene block copolymers are disclosed in the German Patent 16 94 366. The patent claims a method for the preparation of polyurethane foams which is characterized that polyoxyalkylene polysiloxane block copolymers are used, the polysiloxane block of which, however, is built up in a known manner and the polyoxyalkylene block of which consists of 25 to 70% by weight of a polyoxyalkylene with an average molecular weight of 1,600 to 4,000 and an ethylene oxide content of 20 to 100% by weight, the remainder being propylene oxide and optionally higher alkylene oxides, and 30 to 75% by weight of a polyoxyalkylene with an average molecular weight of 400 to 1,200 and an ethylene oxide content of 65 to 100% by weight, the remainder being propylene oxide and optionally higher alkylene oxides. The essence of this patent thus is that polyoxyalkylene blocks of defined, different construction and, with that, of different hydrophilicity, are contained in the block copolymer.